This invention relates in general to a part or machine component having a lubricated wear surface and, more particularly, to a process for providing a powdered metal machine component with a lubricated wear surface, to the machine component itself, and to a bearing having such a component.
Powdered metal machine components are rarely compacted to theoretical density, which is of course the density of the wrought metal, and as a consequence such components contain pores. Where the component carries a wear surface against which another component moves, these pores serve a very useful purpose in that they will hold a liquid lubricant and release it at the wear surface so as to minimize friction between the wear surface and the other component. Indeed, the pores may serve as a lubricant reservoir capable of supplying lubrication to the wear surface in the event the wear surface looses its normal supply of lubrication.
U.S. Pat. No. 4,601,592, granted to The Timken Company on Jul. 22, 1986, discloses a tapered roller bearing having a rib ring which is formed from powdered steel. This rib ring provides a wear surface along which the large ends of the tapered rollers move, thus preventing the rollers from being expelled from the space they occupy between the two races of the bearing. The ring holds a lubricant in its pores, and those pores are open and exposed at the wear surface. Should the bearing lose its normal supply of lubrication, the ring will release its lubricant to the wear surface and prevent failure of the bearing at this critical surface.
A powdered metal part, however, requires special preparation before its pores will release a stored lubricant at a wear surface on the part. This derives from the fact that the compaction of powdered metal into the shape desired for the part and the subsequent sintering to retain that shape does not proceed with the precision required for many parts. As a consequence, the compacted and sintered part must undergo further mechanical machining, such as grinding, to bring it to the dimensions, shape and surface finish suitable for service. The machining obliterates and closes the pores, producing a so-called "Bielby layer" which is undesirable at the wear surface, since it prevents a lubricant within the ring from being released at that surface.
U.S. Pat. No. 4,601,592 addresses this problem by recommending that the powdered metal part be chemically etched along its wear surface. This eliminates the Bielby layer and opens the pores at the wear surface so that a lubricant may be introduced into the wear ring and likewise pass out of the wear ring at the wear surface when needed. But the etchant used to remove the Bielby layer also enters the pores where it is difficult to remove and more significantly causes unwanted corrosion. Indeed, the etchant together with the grinding fluid, which is primarily water and is likewise absorbed into the pores during grinding, produces a particularly corrosive combination.
Impregnating the pores with oil before the grind would keep out most of the grinding fluid and afterwards the etchant as well, but oil inhibits etching and is likely to cause an uneven etch. Thus, the pores are kept free of oil, at least until completion of the etching.
The present invention resides in a process for removing the Bielby layer from a powdered metal machine component by subjecting the component to erosion at the surface areas where the Bielby layer is to be eliminated, preferably erosion by electro-discharge machining. The process is particularly useful for providing a powdered metal machine component, which has been mechanically machined, with wear surface against which another machine element moves, in that the pores of the powdered metal component are open at the wear surface and thus can release lubricant which is stored in the pores, so that the other element moves along the wear surface with minimal friction. The invention also resides in a machine component having a wear surface formed by the process and that component may be a rib for guiding and confining rolling elements of an antifriction bearing. Indeed, the invention further resides in a bearing having such a rib.